Fiore is the secondary antagonist of Sailor Moon R: The Movie (which includes "The Promise of the Rose" in the English dub's title). "Fiore" is Italian for "flower."
He was a lonely alien who took great strength from his friendship with Mamoru Chiba (Darien in English) and spent many years looking for a flower for him, as promised, since Mamoru gave him a rose before he vanished, much to Mamoru's sorrow. He vanished because he found the Earth's atmosphere unacceptable. This quest for a flower put him in the path of the Xenian Flower, which caused a thought to strike him: he vowed that nobody but himself would give Mamoru any flowers, and he changed into a bad guy, seeing the Sailor Scouts as the negative forces.
While Fiore was trying to take Sailor Moon's crystal, she showed him how Mamoru became her friend (as Usagi [or "Serena" in the English dub]) when she was a little girl (the source of how Mamoru got his rose). This caused the crystal to change, and Fiore to feel remorse and hesitate to take the crystal, much to the Xenian Flower's dismay.
At the end, when Sailor Moon is dead, Fiore fulfills his childhood promise by giving Mamoru a flower, which happens to be his own life. That is the Friendship Flower whose nectar he (turning back to his juvenile form) tells him to give to Sailor Moon. He then goes away by bubble after this is done.